My two distro recommendations for "you install it and then never need to touch the thing again" are Mint and Debian. I installed the latter for my father (who's technical knowledge only really extends as far as "knows or can figure out how to put in the wifi password on most devices") several years ago when he wanted to get away from windows, and the only real issue since was the laptop *physically getting stepped on* at one point and thus needing a replacement screen.
Yeah, debian stable doesn't come with the latest and greatest versions of everything, but for the vast majority of people it doesn't *need* the latest and greatest versions of software. It just needs to *work*...